Patricia Hewitt in 1975, when she was working for the National Council for Civil Liberties. Photograph: BBC

The members' hotline for the Paedophile Information Exchange (PIE) rang, of all places, inside the Home Office. The phone would be picked up by Steven Smith, a paedophile and member of the group who worked in security in Whitehall, from where he would tell callers where to go for the next meeting to discuss issues including decriminalising sex with children as young as four.

The brazen tactic was one of many employed in the 1970s by the long defunct organisation that has now embroiled a number of senior Labour figures in a row over an alleged link between civil liberties campaigning four decades ago and endorsement of paedophilia.

This week Harriet Harman, the Labour deputy leader, who was legal adviser at the National Council for Civil Liberties (NCCL) from 1978 until 1982, and her husband Jack Dromey, currently an opposition policing spokesman, who was on the NCCL executive from 1970 to 1979, were forced to defend their organisation's affiliation with PIE, which ran for eight years from 1975 to 1983. Dromey and Harman have denied offering any support or endorsement for what they call a "vile" organisation.

But evidence of links between the NCCL and PIE continued to emerge. Archive documents appear to show how the paedophile group managed to influence policy at the civil liberties group despite being run by people who believed in their right to have sex with young children.

According to archives held at Hull University, in December 1975 Keith Hose, chairman of PIE, wrote to Patricia Hewitt, then general secretary of NCCL and later a Labour health secretary, asking her to consider PIE's views in its policy on ages of consent. The letter was on PIE notepaper which features a logo of two bare-legged children sitting on a rock. Hewitt wrote back saying: "We have found your evidence ... most helpful and I think it has certainly been taken into account by the people preparing our evidence."

Hewitt did not respond to requests for comment and has yet to issue any response to the growing evidence of links between the organisations.

Earlier this week convicted paedophile, Tom O'Carroll told The Guardian he remained a member of the NCCL's gay rights committee for several years after Harman claimed his organisation was marginalised. NCCL archives in London have also shown how O'Carroll, a former chairman of PIE, asked Nettie Pollard, a staff member at the organisation, about the possibility of amendments to the 1978 child protection bill.

The archive includes a NCCL briefing note that argues that child pornography should not be outlawed in the bill and how Harman, who started at NCCL in 1978, herself urged changes to the bill that year telling MPs "images of children should only be considered pornographic if it could be proven the subject suffered".

She has said the submission simply argued for some amendments to guard against unintended consequences, including parents being criminalised for taking pictures of their children on the beach or in the bath. A spokesman for Harman has insisted neither O'Carroll nor Pollard influenced her work. Harman has said she regretted that "this vile organisation ever existed and that it ever had anything to do with NCCL", while Dromey said he in fact "took on" PIE saying he was "at the forefront of repeated public condemnations of PIE and their despicable views".

PIE was not only a campaign group. Its members formed paedophile networks, sharing images and contacts with each other and at least seven of its number went on to be jailed for paedophilia offences ranging from sex with a minor to distributing indecent images.

Peter Hain clashed with PIE in 1975 when he was honorary vice-president of the Campaign for Homosexual Equality. PIE's founding chairman, Keith Hose, had persuaded the gay rights group not to relegate paedophilia to a minor issue at its conference and it was too much for Hain at the time.He fired off a letter to CHE: "Some plain speaking is called for: paedophilia is not a condition to be given a nod and a wink as a healthy fringe activity in society – it is a wholly undersireable abnormality requiring sensitive treatment." Recalling the dispute , Hain said: "There was a kind of loose trendiness around the debate that horrified and appalled me. It was crossing a boundary that should never have been crossed in my view."

Its early members were drawn from the gay rights movement because they "were interested in man-boy relationships", said O'Carroll. There were around 300 members at any one time. PIE was born in part from the reforms of Harold Wilson's home secretary, Roy Jenkins, who decriminalised homosexual acts between men aged 21 and over. "[Jenkins] created within the Home Office an atmosphere of excitement around liberal reform," O'Carroll said.

They felt a moment was coming when their desire for "mutual and loving relationships" between adults and children would shame them no more. But drumming up members was hard because so few publications would take their adverts and PIE sought affiliation with NCCL "to create a greater weight of opinion by joining together in a common casue", O'Carroll said. "It got me a platform or two."

According to archives, Larry Gostin, chief executive of NCCL in the early 1980s, told one member who was upset about the affiliation "we have such a profound repugnance of their aims but we have always maintained that affiliation to us does not in any way denote our agreement with the aims of the organisation". He continued: "NCCL has always, quite rightly, prided itself on defending the right to speak although we very much disagree with the view expressed."

Warren Middleton, vice chairman of PIE said in 1975 its goal was to stop people "cowering with fear and revulsion at the mention of the word paedophile" and said paedophiles were "neither sick nor evil, at least no more than the remainder of society, but lonely misunderstood individuals ostracised by society which has made little attempt to understand them."

O'Carroll said that some members sharing images including of children having sex with adults. "We made a terrible mistake," he said. "We were trying to be a political organisation and at the same time we were trying to be a support organisation."

He said he believed Hewitt, Harman and Dromey "disapproved of us behind our backs" but "bit their lip on the subject". "They were not hoping to keep in with me or PIE but other people on the left, such as Gay Left and Gay Liberation front," he said. "In fairness to the three of them, their position is not fairly represented by the Daily Mail's attack," he said. "I think they were more interested in establishing themselves, gaining credibility and advancing in their political careers than either pro-or anti-paedophile positions."